Mobile phones generally use vibration as a means for informing the user of incoming calls. The silent alert provided by vibration is helpful in instances where an alerting sound is inappropriate, or where an alerting sound is inadequate. Thus, tactile sensation is used to notify the mobile user of calls or other events, such as alarms, calendar reminders, etc.
Many mobile phones use eccentric mass vibrator motors for silent alerts. Eccentric mass vibrator motors are driven at a single direct current (DC) voltage in mobile phone handsets. Internal variation in the vibrator motor may cause the speed of one motor (at the phone's drive voltage) to rotate at a different speed than another vibrator motor from the same manufacturer. In fact, as eccentric mass vibrators are used, they may wear, which causes the motors to vary from an initial speed.
Humans are sensitive to different frequency ranges of vibration. Vibration increases as rotational speed increases. Beyond a certain rotational speed, however, the human perception of vibration diminishes. Thus, if a target rotational speed can be identified for a given vibrator/phone, it would be optimum if the vibrator motor functioned at or near this rotational speed.